1. Field of the Invention
The present invention relates to sensors for detecting a physical magnitude comprising a mechanical resonator which is caused to vibrate and to which a stress is applied representative of the magnitude to be measured.
2. Description of Related Art
In sensors of this type, the resonance frequency of the resonator varies as a function of the stress which is applied thereto, measurement of this resonance frequency being representative of the physical magnitude to be measured which may be in particular a pressure, an acceleration, a temperature.
Traditionally, the resonator is made from a piezoelectric material with electrodes fed at the resonance frequency, an automatic control maintaining this resonance.
Sensors of the above defined type have the combined advantages of great stability and high accuracy and they are used particularly in the aeronautics field and mainly for measuring pressures which are parameters to be taken into account for determining the flight parameters of an aircraft, necessary for piloting it.
The evolution of aeronautics techniques has led to an increase in the number of such sensors which are distributed at different positions on the aircraft, as close as possible to the pressure take-off and their information is transmitted by electric connections to the user means, for example an on-board computer.
The major disadvantage of this type of sensor, with electric internal operation principle and electric connection resides in their susceptibility to electromagnetic radiation, from the simple parasite which disturbs the information to lightning or an electromagnetic pulse (IEM) which may cause destruction of the vital parts of the aircraft and this is mainly the case for the new generations of electric flight control apparatus.
To overcome this drawback, a number of constructions for replacing the electric conductors by optical fibers have already been proposed, thus making it possible to offset the control and measurement means in the aircraft, particularly the electric oscillation circuit of the resonator and to provide total galvanic isolation for the sensor.
Thus, it is known to associate an electro-optical transmitter with a traditional electric sensor for transmitting the information.
It is also known to remotely feed the sensor. The electric energy required for operating a sensor comes from light energy transformed by a photodiode and transmitted by a first optical fiber. The measurement signal, after transformation by an emissive diode, is itself transmitted by a second optical fiber.
It is further known to use directly an optical fiber as sensor means, this being for example the case in microcurvature sensors.
These latter solutions have however the drawbacks of leading to high performance but complex sensors and involving in any case an electric implementation of the resonator, or to sensors having low metrological qualities making them incompatible with the performances required.
The Applicant has already proposed, in a French patent application not yet published, exciting the resonator by photo-thermal effect, obtained by a light beam from an emissive diode, thermo-elastic coupling, such as taught by the article "Electronics Letters", Apr. 29, 1982, vol 18, n. 3, pages 381, 382, transforming the thermal energy into a stress causing the resonator to vibrate and detection of the resonance frequency being based on the principle of bi-refringence, after a detection light beam, not modulated at emission, has passed through the resonator and the analysis of which is effected by an opto-electronic transducer. Photo-elastic or photo-acoustic resonators of this latter type could equip a sensor network. However, the resonators of the network could not be excited simultaneously without having in particular separate oscillation electric circuits, i.e. one for each sensor.
The present invention, which aims at providing a resonator satisfying all the above recalled requirements is based on the slightly different concept of a photo-acoustic oscillator introduced in the article by R. M. Longdon and D. L. Dowe "Photoacoustic oscillator sensors", Proc. SPIE Conf., Fibre Optic Sensors II, The Hague, 1987 pages 867-893. It is taught therein to form a Fabry Perot optical cavity between a face of the resonator and the output of an optical fiber, the movement of the resonator in the field of standing waves prevailing in the optical cavity causing modulation of the intensity of the light radiation which, in its turn, maintains the oscillations of the resonator.
However, such a resonator has the further drawback of requiring relative positioning of the resonator and fiber which must be extremely precise.